To provide an image forming apparatus capable of forming a high-quality output image over a long term by preventing the deterioration of the quality of the output image.

The image forming apparatus equipped with a developing means for electrostatically sticking developer containing toner to an image carrier is provided with a developer collecting means for collecting the developer, a developer accumulating means for accumulating the developer collected by the developer collecting means, a developer electrification amount measuring means for measuring the electrification amount of the developer, a developer mass measuring means for measuring the mass of the developer, and a triboelectrified amount detecting means for detecting the triboelectrified amount (charge amount per unit mass) of the developer from the electrification amount obtained by the electrification amount measuring means and the mass obtained by the mass measuring means. Then, the developer collecting means and the developer accumulating means are arranged near the developing cylinder of the developing means.